It’s been a long time since a world leader has publicly threatened to use nuclear weapons, but Vladimir Putin has just done so, warning in a speech that if anybody dares to use military measures to attempt to stop Russia’s annexation of Ukraine, he has the weapons.

The warning may have been hollow, only a display of the Russian president’s teeth, but it was noted. It sparked nightmare scenarios in which Putin’s objectives in Ukraine may accidentally or miscalculately lead to nuclear war.

“In terms of military affairs, even after the disintegration of the Soviet Union and the loss of a significant portion of its capabilities, Russia remains one of the most powerful nuclear powers today,” Putin said in his pre-invasion speech on Thursday.

“It also has a competitive advantage in a number of cutting-edge weaponry.” In this scenario, there should be no question in anyone’s mind that any prospective aggressor who attacks our nation directly would be defeated and suffer terrible repercussions.”

By even implying a nuclear reaction, Putin raised the troubling potential that the present conflict in Ukraine may devolve into an atomic clash between Russia and the US.

Those who grew up during the Cold War, when American schoolchildren were advised to dive and cover beneath their desks in case of nuclear alarms, are familiar with that horrific scenario.

However, with the collapse of the Berlin Wall and the breakup of the Soviet Union, when the two countries appeared to be on a glide path to disarmament, democracy, and prosperity, the threat faded from the popular mind.

Prior to that, even children understood the horrific concept of mutual assured destruction, or MAD, a balance in nuclear capabilities designed to keep each side’s hands off the nuclear button, knowing that any use of doomsday weapons would result in the extinction of both sides in a battle.

And, astonishingly, no government has deployed nuclear weapons since 1945, when President Harry Truman launched bombs on Japan in the hopes of fast ending World War II. It did, but at a cost of nearly 200,000 deaths in Hiroshima and Nagasaki, largely civilians. Even now, many people see it as a crime against humanity and wonder if it was worth it.

The United States enjoyed a nuclear monopoly for a brief while after the war. However, the Soviet Union declared its own nuclear bomb a few years later, and the Cold War’s two sides began an arms race over the following few decades to construct and manufacture increasingly stronger weapons.

With the dissolution of the Soviet Union in 1991 and Boris Yeltsin’s transition of the country into a hoped-for democracy, the US and Russia agreed to reduce their weaponry. Following the dissolution of the Soviet Union, several post-Soviet nations such as Ukraine, Kazakhstan, and Belarus voluntarily handed up nuclear weapons on their soil.

If nuclear weapons have been mentioned at all in recent years, it has typically been in the context of preventing their spread to nations like North Korea and Iran. (Iran denies wanting them, and North Korea has been steadily but gradually developing both nuclear weapons and delivery systems.) Since the beginning of the Ukraine crisis, President Joe Biden has been aware of the threat of nuclear conflict between Russia and NATO. He has stated from the beginning that NATO will not send troops into Ukraine because doing so would result in direct confrontation between the US and Russia, perhaps leading to nuclear escalation and World War III.

It was an implicit acknowledgement that the US would not go to war with Russia over Ukraine, preferring instead to use severe sanctions to choke Russia’s economy.

But there was another truth in the admission. Ukraine remained on its own in the face of a Russian invasion since it is a non-treaty member and does not qualify for protection under NATO’s nuclear umbrella.

However, if Putin were to attack one of America’s NATO allies, the situation would be different, since the agreement is totally dedicated to mutual defense, as Biden has stated.

Why would Putin even bring it up in his address, knowing that Biden had previously ruled out a military response?

He may have done so in part to catch the West off guard, preventing it from intervening aggressively to defend Ukraine from Putin’s blitzkrieg takeover attempt.

The broader background, however, seems to be his strong desire to demonstrate to the world that Russia is a powerful nation that should not be overlooked. Putin frequently mentions Russia’s humiliation following the demise of the Soviet Union. He mirrored the Soviet Union’s bravado in staring down the United States and earning respect in his mind by swinging his nuclear sword.

Putin’s veiled threat to deploy nuclear weapons against any country that intervened in Ukraine elicited only a subdued response from Pentagon officials following his address.

U.S. authorities “don’t perceive an elevated threat in that sense,” a senior defense official said Thursday, speaking on the condition of anonymity to discuss internal discussions.

Putin’s words grate on the Pentagon’s nerves because they underline a long-held fear that he could be willing to deploy nuclear weapons in Europe prematurely in the event of a crisis.

This is one of the reasons Washington has attempted unsuccessfully for years to persuade Moscow to accept restrictions on so-called tactical nuclear weapons, which have a shorter range and may be deployed in a regional conflict. Russia has a significant numerical advantage in this area, and some authorities believe the disparity is widening.

When Russia’s troop buildup in Ukraine reached a crisis level this month, the Biden administration was wrapping up a Nuclear Posture Review –– a study of prospective modifications to US nuclear capabilities and the regulations that govern their deployment. It’s unclear if the findings of that research will be revised in light of Russia’s invasion.